Through the hydrologic cycle, natural landscapes, such as forest, fields, and wetlands, allow rainwater and snowmelt to slowly percolate into the ground, which then recharges the groundwater table. In contrast, urban surfaces such as paving, buildings, and stoned walkways prevent water from filtering into the ground. As a result, water remains on the surface, accumulates, and runs over the surface creating the potential for flooding and erosion. As the water flows, it becomes contaminated by the waste on the paved surface such as oil and gas, road salts, dog droppings and even pesticides, and then flows directly into storm sewers which are connected to our waterways.

Adapted from Pollution Probe's Source Water Protection Primer